Northern leaf blight (NLB) caused by the fungus Setosphaeria turcica (also known as Exserohilum turcicum or Helminthosporium turcicum) is a major disease of maize in North America, South America, Africa and Asia. Symptoms can range from cigar-shaped lesions on the lower leaves to complete destruction of the foliage, thereby reducing the amount of leaf surface area available for photosynthesis which in turn impacts grain yield. Disease management strategies include crop rotation, destruction of old maize residues by tillage, and fungicide application, all of which are aimed at reducing the fungal inoculum. However, the most effective and most preferred method of control for northern leaf blight is the planting of resistant hybrids.
Several varieties or races of Exserohilum turcicum are present in nature, leaving growers with two hybrid options: partial resistant hybrids, which offer low-level, broad spectrum protection against multiple races, and race-specific resistant hybrids, which protect against a specific race. Genetic sources of resistance to Exserohilum turcicum have been described, and four Exserohilum turcicum (previously called Helminthosporium turcicum) resistance loci have been identified: Ht1, Ht2, Ht3, and Htn1. Gene Ht1 maps to the long arm of chromosome 2 where it is closely linked to umc36 (Coe, E. H. et al. (1988), Corn and Corn Improvement, 3rd edn., pp. 81-258), sgcr506 (Gupta, M. et al. (1989) Maize Genet. Coop. Newsl. 63, 112), umc150B (Bentolila, S. et al. (1991) Theor. Appl. Genet., 82:393-398), and pic18a (Collins et al. (1998) Molecular Plant-Microbe Interactions, 11:968-978), and it is closely flanked by umc22 and umc122 (Li et al. (1998) Hereditas, 129:101-106). Gene Ht2 maps to the long arm of chromosome 8 in the umc48-umc89 interval (Zaitlin et al. (1992) Maize Genet. Coop. Newsl., 66, 69-70), and gene Ht3 maps to chromosome 7 near bnlg1666 (Van Staden, D et al. (2001) Maize Genetics Conference Abstracts 43:P134). The Htn1 gene maps to chromosome 8, approximately 10 cM distal to Ht2 and 0.8 cM distal to the RFLP marker umc117 (Simcox and Bennetzen (1993) Maize Genet. Coop. Newl. 67, 118-119; Simcox and Bennetzen (1993) Phytopathology, 83:1326-1330; Chung et al. (2010) Theor App Gen Epub).
Since the QTL respond to different races and each QTL has a variable effect on the northern leaf blight resistance trait, it is desirable to identify new sources of genetic resistance that can be combined with other known resistance loci to enhance overall resistance to northern leaf blight.